Gravure printing method

ABSTRACT

Provided is a gravure printing method by which high printing density and excellent highlight suitability are obtained, even if a gravure plate having high resolution and reduced thickness is used. The gravure printing method comprises: using an aqueous ink having a Zahn cup #3 viscosity at 20° C. of 11.0 seconds or more and 20.0 seconds or less, and having an evaporation rate of 30 mass % or less in a drying test, the drying test involving drying 1 g of the ink at a temperature of 40° C. and an air flow of 1,400 L/min for 30 minutes; and transferring 1 ml/m2 or more and 7 ml/m2 or less of the ink onto a printing medium.

TECHNICAL FIELD

The present invention relates to a gravure printing method using anaqueous ink.

BACKGROUND ART

Gravure printing involves forming gravure cells corresponding to platemaking information on a cylinder serving as a plate base material toprepare a plate surface, filling each of the cells with a gravure ink,and transferring the gravure ink onto a material to be printed.

In recent years, along with development of an environment-consciousproduct, an aqueous gravure ink with reduced VOCs has been put intopractical use. However, the aqueous gravure ink is inferior in dryingproperty to an oil-based gravure ink. Therefore, in order to reduce anamount of a vehicle of the ink to be dried, the aqueous gravure ink hasrequired a gravure cell depth of about 14 μm, which is shallow ascompared to that in the case of the oil-based gravure ink, i.e., about20 μm, so as to reduce a transfer amount of the ink.

In order to solve such problem, a production method for a plate makingroll and an aqueous gravure ink have been proposed.

For example, in Patent Document 1, there is disclosed a gravure platemaking roll including a surface reinforcing coating layer, which has notoxicity and eliminates a concern about occurrence of pollution.

In Patent Document 2, there is a disclosure that a satisfactory dryingproperty and printing suitability are both achieved through use of anaqueous gravure printing ink composition containing 1 wt % to 10 wt % ofan organic solvent.

PRIOR ART DOCUMENTS Patent Document Patent Document 1: JP 2007-125730 APatent Document 2: JP 2002-188029 A SUMMARY OF THE INVENTION Problems tobe Solved by the Invention

However, in Patent Document 1, a gravure cell depth is specified to befrom 5 μm to 150 μm and a specific aqueous ink is explicitly described,but its ink composition is not mentioned.

In the ink composition of Patent Document 2, an ink is specified, butthe number of gravure lines is not explicitly stated and a cell depthused in Examples is as deep as 20 μm.

In view of the foregoing, an object of the present invention is toprovide a gravure printing method by which high printing density andexcellent highlight suitability (reproducibility of a printed portionhaving a low halftone dot area ratio) can be obtained even if a gravureplate having high resolution (e.g., from 150 lines/inch to 350lines/inch) and reduced thickness (e.g., from 3 μm to 15 μm) is used.

Means for Solving Problems

As a result of extensive investigations, the inventors of the presentinvention have found that the above-mentioned object can be achieved bya gravure printing method, including: using an aqueous ink having a Zahncup #3 viscosity at 20° C. of 11.0 seconds or more and 20.0 seconds orless, and having an evaporation rate of 30 mass % or less in a dryingtest (involving drying 1 g of the ink at a temperature of 40° C. and anair flow of 1,400 L/min for 30 minutes); and transferring 1 ml/m² ormore and 7 ml/m² or less of the ink onto a printing medium.

Specifically, it is considered that, when the aqueous ink having a Zahncup #3 viscosity at 20° C. of 11 seconds or more and 18 seconds or less,and an evaporation rate of 30 mass % or less in the drying test(involving drying 1 g of the ink at a temperature of 40° C. and an airflow of 1,400 L/min for 30 minutes) is used, drying in the cells of agravure plate having high resolution and reduced thickness can besuppressed to allow the transfer ratio of the ink onto a printingsubstrate to be kept high even for the ink in cells each having a smallvolume, and hence even a dot smaller than that in the related art can betransferred, resulting in satisfactory highlight suitability.

It is considered that, when an aqueous ink having a Zahn cup #3viscosity at 20° C. of 11.0 seconds or more and 20.0 seconds or less,and an evaporation rate of 30 mass % or less in the drying test(involving drying 1 g of the ink at a temperature of 40° C. and an airflow of 1,400 L/min for 30 minutes) is used, the drying in the cellsproceeds to lower the transfer ratio of the ink, and hence the highlightsuitability is poor.

It is preferred that, in the gravure printing method according to oneembodiment of the present invention, a transfer ratio of the ink from agravure cell onto the printing medium be 50% or more.

It is preferred that, in the gravure printing method according to oneembodiment of the present invention, a gravure cell have a volume of 2ml/m² or more and 8 ml/m² or less.

It is preferred that, in the gravure printing method according to oneembodiment of the present invention, a gravure cell have a depth of 3 μmor more and 15 μm or less.

It is suitable that, in the gravure printing method according to oneembodiment of the present invention, a gravure plate number be 150lines/inch or more and 350 lines/inch or less.

Advantageous Effects of the Invention

According to the present invention, by the gravure printing method,including: using an aqueous ink having a Zahn cup #3 viscosity at 20° C.of 11.0 seconds or more and 20.0 seconds or less, and having anevaporation rate of 30 mass % or less in a drying test (involving drying1 g of the ink at a temperature of 40° C. and an air flow of 1,400 L/minfor 30 minutes); and transferring 1 ml/m² or more and 7 ml/m² or less ofthe ink onto a printing medium, high printing density and excellenthighlight suitability can be obtained even if a gravure plate havinghigh resolution and reduced thickness is used.

MODES FOR CARRYING OUT THE INVENTION

[Gravure Plate]

As a gravure plate that allows printing to be performed using a gravureprinting method of the present invention, any gravure plate made by ahitherto known plate making method may be applied. For example, thegravure printing method of the present invention may be applied to agravure plate made by a plate making method such as a conventionalmethod, a halftone gravure method, or an electronic engraving method.

In the conventional method or the halftone gravure method, cells may beformed through the application of a photosensitive film, exposure tolight, development, and etching (etching method). The electronicengraving method is a method involving mechanically engraving cellsdirectly on the cylinder with an engraving needle. In the method offorming cells by the electronic engraving method, the cells are formedas square pyramids, and hence the transfer of an ink in a highlightedportion is satisfactory. In the etching method, the cells are formed asshallow dish-like recesses, and hence the transfer of the ink is poorerowing to clogging of the ink in the cells in the highlighted portion, inwhich the cells are extremely small, than in the electronic engravingmethod. However, in the etching method, the cells are formed so as toenable ink flow at intersections of screen lines in the most shadowyportion, and hence the etching method has advantages in that the ink canbe transferred reliably at the intersections and the outline of a lettercan be an outline without serration. Further, the cells in the mostshadowy portion are also shallow, and hence the etching method is suitedfor printing using an aqueous gravure ink. In addition, a gravure platemade by a laser plate making method involving exposing a cylinder tolight through the use of a laser beam is particularly suited for thegravure printing method of the present invention. This is because thegravure printing method of the present invention can provide highprinting density and excellent highlight suitability and hence the platemaking is suitably performed by the laser plate making method capable ofproviding a high-resolution gravure plate.

From the viewpoints of the printing density and a drying property, thevolume of each of the gravure cells is preferably 2 ml/m² or more and 8ml/m² or less, more preferably 2.5 ml/m² or more and 7 ml/m² or less,still more preferably 3 ml/m² or more and 6.5 ml/m² or less.

From the viewpoints of the printing density and the drying property, thedepth of each of the gravure cells is preferably 3 μm or more and 15 μmor less, more preferably 4 μm or more and 13 μm or less, still morepreferably 5 μm or more and 10 μm or less.

From the viewpoints of the printing density and the drying property, agravure plate number is preferably 150 lines/inch or more and 350lines/inch or less, more preferably 175 lines/inch or more and 300lines/inch or less, still more preferably 200 lines/inch or more and 250lines/inch or less.

When the gravure plate number is less than 150 lines/inch, the dryingproperty may be lowered, and when the gravure plate number is more than350 lines/inch, the printing density may be lowered.

The shape of each of the cells is not particularly limited, and anypattern, such as a straight line, a curve, an arc, a zigzag shape, ahelical shape, a lattice shape, a honeycomb shape, a rhombic shape, atriangular shape, a quadrilateral shape, or a geometric pattern shape,may be preferably used. A tone is expressed by changing the distributednumber of such cells from 100% to 0% continuously, or continuously witha certain interval (e.g., at an increment of 10%).

[Printing Method]

Gravure printing involves: supplying an ink to the surface of a gravurecylinder having cells formed in its surface while rotating the gravurecylinder; scraping off the ink with a doctor blade fixed at apredetermined position to leave the ink only in the cells; and bringinga continuously supplied printing medium into pressure contact with thegravure cylinder through the use of an impression drum having a surfaceformed of rubber to transfer only the ink in the cells of the gravurecylinder onto the printing medium, to thereby print a pattern.

From the viewpoints of the printing density and the drying property, theamount of the ink to be transferred onto the printing medium ispreferably 1 ml/m² or more and 7 ml/m² or less, more preferably 1.5ml/m² or more and 6 ml/m² or less, still more preferably 2 ml/m² or moreand 5.6 ml/m² or less. It is particularly preferred that the amount ofthe ink to be transferred onto the printing medium when measured in a100% halftone dot printed portion fall within the above-mentioned range.

From the viewpoint of the highlight suitability, the transfer ratio ofthe ink from each of the gravure cells onto the printing medium ispreferably 50% or more and 100% or less, more preferably 55% or more,still more preferably 60% or more. The transfer ratio may be determinedby the transfer amount of the ink to be transferred onto the printingmedium with respect to the gravure cell volume. It is particularlypreferred that a transfer ratio when measured for a cell in a 100%halftone dot printed portion fall within the above-mentioned range.

The pressure of the doctor blade may be adjusted to the extent that theink can be normally scraped off and fogging does not occur. In addition,as a material for the doctor blade, a ceramic material, which is lessliable to wear, may be used as well as a general stainless-steelmaterial.

The drying of the ink may be adjusted on the basis of a temperature andan air flow that do not damage the printing substrate.

A printing speed may be increased to the extent that a guide roll is notstained and the offset of the ink after take-up does not occur.

For printing on a resin film for food packaging, the following systemsare available: a surface printing system involving performing printingonly on a surface of the resin film corresponding to a surface of a bag;and a method (reverse printing system) involving performing printing ona surface of the resin film corresponding to a surface opposite to asurface of a bag, and further laminating another film on the printingsurface. The aqueous ink to be used in the present invention may beapplied to any of the surface printing system and the reverse printingsystem.

[Aqueous Ink]

The aqueous ink to be used in the present invention preferably containsa pigment, a polymer, a water-soluble organic solvent, a surfactant, andwater. “Parts by mass” and “mass %” are each a value in a solid contentunless otherwise specified.

The Zahn cup #3 viscosity at 20° C. of the aqueous ink is 11.0 secondsor more and 20.0 seconds or less. From the viewpoint of thetransferability of the ink, the Zahn cup #3 viscosity at 20° C. ispreferably 11.5 seconds or more, more preferably 12.0 seconds or more,still more preferably 13.5 seconds or more, and is preferably 19.0seconds or less, more preferably 18.0 seconds or less, still morepreferably 17.5 seconds or less.

In addition, from the viewpoint of the transferability of the ink, theZahn cup #3 viscosity at 20° C. of the aqueous ink is 11.0 seconds ormore and 20.0 seconds or less, preferably 11.5 seconds or more and 19.0seconds or less, more preferably 12.0 seconds or more and 18.0 secondsor less.

From the viewpoint of the highlight suitability, the aqueous ink is anink having an evaporation rate of preferably 30 mass % or less, morepreferably 25 mass % or less, still more preferably 20 mass % or less ina drying test (involving drying 1 g of the ink at a temperature of 40°C. and an air flow of 1,400 L/min for 30 minutes). The viscosity of theaqueous ink may be adjusted on the basis of the kinds and contents ofthe polymer, the water-soluble organic solvent, the surfactant, and thelike to be blended.

[Pigment]

The aqueous ink to be used in the present invention may contain apigment.

The kind of the pigment to be used in the present invention may be anyof an inorganic pigment and an organic pigment.

Examples of the inorganic pigment include carbon black and a metaloxide. Carbon black is preferred as an inorganic pigment for a blackink. Examples of the carbon black include furnace black, thermal lampblack, acetylene black, and channel black. As an inorganic pigment for awhite ink, there are given, for example, metal oxides, such as titaniumdioxide, zinc oxide, silica, alumina, and magnesium oxide.

Examples of the organic pigment include an azo pigment, a diazo pigment,a phthalocyanine pigment, a quinacridone pigment, an isoindolinonepigment, a dioxazine pigment, a perylene pigment, a perinone pigment, athioindigo pigment, an anthraquinone pigment, and a quinophthalonepigment.

The hue is not particularly limited, and any of chromatic pigments, suchas yellow, magenta, cyan, red, blue, orange, and green, may be used.

The form of the pigment to be used in the present invention is one ormore kinds of pigments selected from a self-dispersible pigment andparticles in which a pigment is dispersed with a polymer.

[Self-Dispersible Pigment]

The self-dispersible pigment that may be used in the present inventionmeans a pigment capable of being dispersed in an aqueous medium withoutuse of a surfactant or a resin, by bonding one or more kinds ofhydrophilic functional groups (an anionic hydrophilic group, such as acarboxy group or a sulfonic acid group, or a cationic hydrophilic group,such as a quaternary ammonium group) to the surface of the pigmentdirectly or via another atomic group, such as an alkanediyl group having1 to 12 carbon atoms. In order to render the pigment into theself-dispersible pigment, for example, a required amount of thehydrophilic functional group may be chemically bonded to the surface ofthe pigment by a conventional method. As commercially available productsof the self-dispersible pigment, there are given, for example: CAB-O-JET200, CAB-O-JET 300, CAB-O-JET 352K, CAB-O-JET 250A, CAB-O-JET 260M,CAB-O-JET 270Y, CAB-O-JET 450A, CAB-O-JET 465M, CAB-O-JET 470Y, andCAB-O-JET 480V manufactured by Cabot Japan K.K.; BONJET CW-1 and BONJETCW-2 manufactured by Orient Chemical Industries Co., Ltd.; Aqua-Black162 manufactured by Tokai Carbon Co., Ltd.; and SENSIJET Black SDP100,SDP1000, and SDP2000 manufactured by Sensient Industrial Colors. Theself-dispersible pigment is preferably used as a pigment aqueousdispersion dispersed in water.

From the viewpoint of printing density, the content of the pigment inthe ink is preferably 1 mass % or more and 20 mass % or less, morepreferably 2 mass % or more and 18 mass % or less, still more preferably3 mass % or more and 15 mass % or less.

[Polymer]

The aqueous ink to be used in the present invention preferably containsa polymer from the viewpoint of improving the dispersion stability ofthe pigment and the viewpoint of improving fixability.

As the polymer to be used in the present invention, any of awater-soluble polymer and a water-insoluble polymer may be preferablyused.

From the viewpoint of dispersing the pigment and the viewpoint of thefixability, the content of the polymer in the ink is preferably 3 mass %or more and 38 mass % or less, more preferably 5 mass % or more and 30mass % or less, still more preferably 5 mass % or more and 25 mass % orless.

[Water-Soluble Polymer]

The water-soluble polymer refers to such a polymer that, when thepolymer is dried at 105° C. for 2 hours to reach a constant mass and isthen dissolved in 100 g of water at 25° C., its dissolution amount is 10g or more.

In the case of an anionic polymer, the dissolution amount is adissolution amount when the anionic groups of the polymer are 100%neutralized with sodium hydroxide.

The water-soluble polymer to be used in the present invention may beused for the purpose of dispersing the pigment in the water-based ink.

Examples of the polymer to be used include polyester, polyurethane, anda vinyl-based polymer. Of those, from the viewpoint of the dispersionstability of the pigment, a vinyl-based polymer obtained by additionpolymerization of a vinyl monomer (a vinyl compound, a vinylidenecompound, or a vinylene compound) is preferred.

Examples of the vinyl-based polymer include acrylic resins andstyrene-acrylic resins, such as “Joncryl 690”, “Joncryl 60”, “Joncryl6610”, and “HPD-71” (all of which are manufactured by BASF Japan Ltd.).

[Water-Insoluble Polymer]

The water-insoluble polymer refers to such a polymer that, when thepolymer is dried at 105° C. for 2 hours to reach a constant mass and isthen dissolved in 100 g of water at 25° C., its dissolution amount isless than 10 g. The dissolution amount is preferably less than 5 g, morepreferably less than 1 g. In the case of an anionic polymer, thedissolution amount is a dissolution amount when the anionic groups ofthe polymer are 100% neutralized with sodium hydroxide.

The water-insoluble polymer may be used by being dispersed in theaqueous ink as polymer particles each containing the pigment and polymerparticles free of the pigment. The water-insoluble polymer containingthe pigment is hereinafter sometimes referred to as “water-insolublepolymer a”, and the water-insoluble polymer free of the pigment ishereinafter sometimes referred to as “water-insoluble polymer b”.

[Water-Insoluble Polymer a]

Examples of the water-insoluble polymer a serving as the polymercontaining the pigment include polyester, polyurethane, and avinyl-based polymer. Of those, from the viewpoint of improving thestorage stability of the water-based ink, a vinyl-based polymer obtainedby addition polymerization of a vinyl monomer (a vinyl compound, avinylidene compound, or a vinylene compound) is preferred.

The vinyl-based polymer is preferably a vinyl-based polymer obtained bycopolymerizing a monomer mixture containing an ionic monomer (a-1)(hereinafter sometimes referred to as “component (a-1)”) and ahydrophobic monomer (a-2) (hereinafter sometimes referred to as“component (a-2)”) (hereinafter sometimes referred to simply as “monomermixture”). This vinyl-based polymer has a constituent unit derived fromthe component (a-1) and a constituent unit derived from the component(a-2).

[Ionic Monomer (a-1)]

The ionic monomer (a-1) is preferably used as a monomer component of thewater-insoluble polymer from the viewpoint of improving the dispersionstability of the pigment-containing polymer particles in the ink.Examples of the ionic monomer include an anionic monomer and a cationicmonomer. Of those, an anionic monomer is preferred.

Examples of the anionic monomer include a carboxylic acid monomer, asulfonic acid monomer, and a phosphoric acid monomer.

Examples of the carboxylic acid monomer include acrylic acid,methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaricacid, citraconic acid, and 2-methacryloyloxymethylsuccinic acid.

Of the above-mentioned anionic monomers, from the viewpoint of improvingthe dispersion stability of the pigment-containing polymer particles inthe ink, carboxylic acid monomers are preferred, and acrylic acid andmethacrylic acid are more preferred.

[Hydrophobic Monomer (a-2)]

The hydrophobic monomer (a-2) is preferably used as a monomer componentof the water-insoluble polymer from the viewpoint of improving thedispersion stability of the pigment-containing polymer particles in theink. Examples of the hydrophobic monomer include an alkyl(meth)acrylate, an aromatic group-containing monomer, and amacromonomer.

The alkyl (meth)acrylate is preferably an alkyl (meth)acrylate having analkyl group having 1 to 22 carbon atoms, preferably 6 to 18 carbonatoms, and examples thereof include methyl (meth)acrylate, ethyl(meth)acrylate, (iso)propyl (meth)acrylate, (iso or tertiary)butyl(meth)acrylate, (iso)amyl (meth)acrylate, cyclohexyl (meth)acrylate,2-ethylhexyl (meth)acrylate, (iso)octyl (meth)acrylate, (iso)decyl(meth)acrylate, (iso)dodecyl (meth)acrylate, and (iso)stearyl(meth)acrylate.

“(Iso or tertiary)” and “(iso)” mean both of a case in which thosegroups are present and a case in which those groups are not present, andrepresent normal in the case where those groups are not present. Inaddition, “(meth)acrylate” represents acrylate and/or methacrylate.

The aromatic group-containing monomer is preferably a vinyl monomerhaving an aromatic group having 6 to 22 carbon atoms that may have asubstituent containing a heteroatom, more preferably a styrene-basedmonomer or an aromatic group-containing (meth)acrylate.

The styrene-based monomer is preferably styrene, 2-methylstyrene, ordivinylbenzene, more preferably styrene.

In addition, the aromatic group-containing (meth)acrylate is preferablybenzyl (meth)acrylate, phenoxyethyl (meth)acrylate, or the like, morepreferably benzyl (meth)acrylate.

The macromonomer is a compound having a polymerizable functional groupat one terminal thereof and having a number-average molecular weight of500 or more and 100,000 or less, and is preferably used as a monomercomponent of the water-insoluble polymer from the viewpoint of improvingthe dispersion stability of the pigment-containing polymer particles inthe ink. The polymerizable functional group present at one terminal ispreferably an acryloyloxy group or a methacryloyloxy group, morepreferably a methacryloyloxy group.

The number-average molecular weight of the macromonomer is preferably1,000 or more and 10,000 or less. The number-average molecular weight ismeasured by a gel permeation chromatography method using, as a solvent,chloroform containing 1 mmol/L dodecyldimethylamine with the use ofpolystyrene as a reference material.

From the viewpoint of improving the dispersion stability of thepigment-containing polymer particles in the ink, the macromonomer ispreferably an aromatic group-containing monomer-based macromonomer or asilicone-based macromonomer, more preferably an aromaticgroup-containing monomer-based macromonomer.

As an aromatic group-containing monomer constituting the aromaticgroup-containing monomer-based macromonomer, the above-mentionedaromatic group-containing monomers are given. Of those, styrene andbenzyl (meth)acrylate are preferred, and styrene is more preferred.

Specific examples of the styrene-based macromonomer include AS-6(S),AN-6(S), and HS-6(S) manufactured by Toagosei Co., Ltd.

An example of the silicone-based macromonomer is an organopolysiloxanehaving a polymerizable functional group at one terminal thereof.

For the hydrophobic monomer (a-2), two or more kinds of theabove-mentioned monomers may be used. The styrene-based monomer, thearomatic group-containing (meth)acrylate, and the macromonomer may beused in combination, and in particular, the macromonomer is preferablyused in combination with another hydrophobic monomer.

[Nonionic Monomer (a-3)]

For the water-insoluble polymer, from the viewpoint of improving thedispersion stability of the pigment-containing polymer particles in theink, it is preferred to further use a nonionic monomer (a-3)(hereinafter sometimes referred to as “component (a-3)”) as a monomercomponent. The nonionic monomer is a monomer having high affinities forwater and a water-soluble organic solvent, and is, for example, amonomer containing a hydroxy group or a polyalkylene glycol.

Examples of the component (a-3) include 2-hydroxyethyl (meth)acrylate,3-hydroxypropyl (meth)acrylate, polyalkylene glycol (meth)acrylates,such as polypropylene glycol (n=2 to 30, n represents an average numberof moles of an oxyalkylene group added. The same applies hereinafter.)(meth)acrylate and polyethylene glycol (meth)acrylate (n=2 to 30),alkoxy polyalkylene glycol (meth)acrylates, such as methoxypolyethyleneglycol (n=1 to 30) (meth)acrylate, and phenoxy (ethyleneglycol-propylene glycol copolymer) (n=1 to 30, ethylene glycol therein:n=1 to 29) (meth)acrylate.

Specific examples of the component (a-3) that is commercially availableinclude: NK Ester TM-20G, TM-40G, TM-90G, and TM-230G manufactured byShin-Nakamura Chemical Co., Ltd.; and BLEMMER PE-90, PE-200, PE-350,PME-100, PME-200, PME-400, PP-500, PP-800, PP-1000, AP-150, AP-400,AP-550, 50PEP-300, 50POEP-800B, and 43PAPE-600B manufactured by NOFCorporation.

The above-mentioned components (a-1) to (a-3) may be used alone or as amixture thereof.

(Content of Each Component or Constituent Unit in Monomer Mixture or inPolymer)

The contents of the components (a-1) to (a-3) in the monomer mixture atthe time of the production of the vinyl-based polymer (contents in termsof unneutralized amounts. The same applies hereinafter.) or the contentsof constituent units derived from the components (a-1) to (a-3) in thewater-insoluble polymer are as described below from the viewpoint ofimproving the dispersion stability of the pigment-containing polymerparticles in the ink.

The content of the component (a-1) is preferably 3 mass % or more and 40mass % or less, more preferably 5 mass % or more and 30 mass % or less,still more preferably 7 mass % or more and 20 mass % or less.

The content of the component (a-2) is preferably 5 mass % or more and 86mass % or less, more preferably 10 mass % or more and 80 mass % or less,still more preferably 20 mass % or more and 60 mass % or less.

The content of the component (a-3) is preferably 5 mass % or more and 60mass % or less, more preferably 10 mass % or more and 55 mass % or less,still more preferably 15 mass % or more and 40 mass % or less.

In addition, the mass ratio “[component (a-1)/component (a-2)]” ispreferably from 0.01 to 1, more preferably from 0.05 to 0.60, still morepreferably from 0.10 to 0.30.

The ionic monomer (a-1) is a dispersing group using electric chargerepulsion in the ink, and the nonionic monomer (a-3) is a dispersinggroup using steric repulsion in the ink. The addition of the nonionicmonomer (a-3) to the hydrophobic monomer (a-2) and the ionic monomer(a-1) further enhances the stability of the pigment-containing polymerparticles in the ink, resulting in higher ink stability.

In addition, when an ink dries, water volatilizes first in many cases.When, in dispersion media (water and water-soluble organic solvents Aand B), the water (dispersion medium that has a high dielectric constantand enhances the electric charge repulsion of a dispersion most amongthe dispersion media) is decreased, the electric charge repulsionproperty of the dispersion is remarkably lowered to lower ink stability,resulting in degradation in ejection property. In this connection, whenthe nonionic monomer is also used (the steric repulsion group isintroduced) in addition to the ionic monomer, a state in which thestability of the pigment-containing polymer particles is high can bekept by the nonionic group (steric repulsion group) even underconditions under which the water volatilizes to lower the dielectricconstant of the dispersion media and hence the electric charge repulsiongroup hardly functions.

(Production of Water-Insoluble Polymer a)

The water-insoluble polymer a is produced by copolymerizing a monomermixture by a known polymerization method. The polymerization method ispreferably a solution polymerization method.

A solvent to be used in the solution polymerization method is notlimited, but is preferably a polar organic solvent, such as an aliphaticalcohol having 1 to 3 carbon atoms, a ketone, an ether, or an ester.Specific examples thereof include methanol, ethanol, acetone, and methylethyl ketone. Of those, methyl ethyl ketone is preferred from theviewpoint of the dissolution property of the solvent for thewater-insoluble polymer.

In the polymerization, a polymerization initiator and a polymerizationchain transfer agent may be used. The polymerization initiator ispreferably an azo compound, more preferably2,2′-azobis(2,4-dimethylvaleronitrile). The polymerization chaintransfer agent is preferably a mercaptan, more preferably2-mercaptoethanol.

Preferred polymerization conditions vary depending on, for example, thekind of the polymerization initiator, but from the viewpoint of thereactivity of the polymerization initiator, a polymerization temperatureis preferably 50° C. or more and 90° C. or less, and a polymerizationtime is preferably 1 hour or more and 20 hours or less. In addition, apolymerization atmosphere is preferably a nitrogen gas atmosphere or aninert gas atmosphere such as argon.

After the completion of the polymerization reaction, the generatedpolymer may be isolated from the reaction solution by a known method,for example, reprecipitation or solvent evaporation. In addition, anunreacted monomer and the like may be removed from the obtained polymerby reprecipitation, membrane separation, chromatography, an extractionmethod, or the like.

From the viewpoint of improving the productivity of an aqueousdispersion of the pigment-containing polymer particles, thewater-insoluble polymer a is preferably used while keeping the form of apolymer solution without the removal of the solvent used in thepolymerization reaction, in order to use the contained organic solventas an organic solvent to be used in a step I to be described later.

From the viewpoint of improving the productivity of the aqueousdispersion of the pigment-containing polymer particles, the solidcontent concentration of the water-insoluble polymer a solution ispreferably 30 mass % or more, more preferably 40 mass % or more, and ispreferably 60 mass % or less, more preferably 50 mass % or less.

From the viewpoint of improving the dispersion stability of thepigment-containing polymer particles in the ink and the viewpoint ofimproving the fixing strength of the ink to a recording medium, theweight-average molecular weight of the water-insoluble polymer a to beused in the present invention is preferably 20,000 or more and 500,000or less, more preferably 30,000 or more and 300,000 or less, still morepreferably 50,000 or more and 200,000 or less.

From the viewpoints of the dispersibility of the pigment and theadsorbability of the polymer, the acid value of the water-insolublepolymer a to be used in the present invention is preferably 50 or moreand 140 or less, more preferably 60 or more and 130 or less, still morepreferably 70 or more and 120 or less.

The weight-average molecular weight and the acid value may be measuredby methods to be described later in Examples.

[Polymer Particles Each Containing Pigment (Pigment-Containing PolymerParticles)]

The pigment-containing polymer particles are particles each having thewater-insoluble polymer adhering to the surface of the pigment, and thewater-insoluble polymer allows the pigment to be stably dispersed inwater and the ink.

(Production of Pigment-Containing Polymer Particles)

The pigment-containing polymer particles can be efficiently produced asan aqueous dispersion by a method including the following step I andstep II.

In the production of the pigment-containing polymer particles by theproduction method including the steps I and II, the pigment and thepolymer a are not chemically bonded to each other, but are in anirreversibly adsorbed state. In the ink, the pigment and the polymer arealways adsorbed to each other, i.e., present as polymer particles eachcontaining the pigment. Meanwhile, “water-insoluble polymer particles”may be used as an ink component as described later, but have thefollowing difference: whereas the pigment-containing polymer particlesare polymer particles each containing the pigment (the pigment and thepolymer are irreversibly adsorbed to each other), the water-insolublepolymer particles are polymer particles free of the pigment.

Step I: a step of subjecting a mixture containing the water-insolublepolymer a, an organic solvent, a pigment, and water (hereinaftersometimes referred to as “pigment mixture”) to dispersion treatment, tothereby obtain a dispersion of pigment-containing polymer particles

Step II: a step of removing the organic solvent from the dispersionobtained in the step I, to thereby obtain an aqueous dispersion of thepigment-containing polymer particles (hereinafter sometimes referred toas “pigment aqueous dispersion”)

In addition, a step III, which is an optional step, may be furtherperformed.

Step III: a step of performing cross-linking treatment by mixing theaqueous dispersion obtained in the step II and a cross-linking agent, tothereby obtain an aqueous dispersion

(Step I)

In the step I, a method involving first dissolving the water-insolublepolymer a in the organic solvent, and then adding and mixing thepigment, water, and as necessary, a neutralizer, a surfactant, and thelike into the resultant organic solvent solution to obtain anoil-in-water dispersion is preferred. The order of addition into theorganic solvent solution of the water-insoluble polymer is not limited,but it is preferred that the water, the neutralizer, and the pigment beadded in the stated order.

The organic solvent for dissolving the water-insoluble polymer a is notlimited, but from the viewpoint of the ease of organic solvent removalin the step II, is preferably an aliphatic alcohol having 1 to 3 carbonatoms, a ketone, an ether, an ester, or the like, more preferably aketone, still more preferably methyl ethyl ketone. When thewater-insoluble polymer is synthesized by the solution polymerizationmethod, the solvent used in the polymerization may be used as it is.Organic solvent-removing treatment is performed in the step II, andhence the above-mentioned organic solvent is not incorporated into finalpigment-containing polymer particles.

When the water-insoluble polymer a is an anionic polymer, anionic groupsin the water-insoluble polymer may be neutralized using a neutralizer.When the neutralizer is used, neutralization is preferably performed soas to achieve a pH of 7 or more and 11 or less. Examples of theneutralizer include bases, such as lithium hydroxide, sodium hydroxide,potassium hydroxide, ammonia, and various amines. In addition, thewater-insoluble polymer may be neutralized in advance.

From the viewpoint of improving the dispersion stability of thepigment-containing polymer particles in each of the ink and the pigmentaqueous dispersion, the degree of neutralization of the anionic groupsof the water-insoluble polymer a is preferably 0.3 mol or more and 3.0mol or less, more preferably 0.4 mol or more and 2.0 mol or less, stillmore preferably 0.5 mol or more and 1.5 mol or less, with respect to 1mol of the anionic groups.

Herein, the degree of neutralization is the quotient of the molarequivalent of the neutralizer by the molar quantity of the anionicgroups of the water-insoluble polymer.

(Content of Each Component in Pigment Mixture)

From the viewpoint of improving the dispersion stability of thepigment-containing polymer particles in the ink for gravure printing andin the pigment aqueous dispersion and the viewpoint of improving theproductivity of the pigment aqueous dispersion, the content of thepigment in the pigment mixture is preferably 10 mass % or more and 30mass % or less, more preferably 12 mass % or more and 27 mass % or less,still more preferably 14 mass % or more and 25 mass % or less.

From the viewpoint of improving the dispersion stability of the pigmentaqueous dispersion and the storage stability of the ink for gravureprinting, the content of the water-insoluble polymer a in the pigmentmixture is preferably 2.0 mass % or more and 15 mass % or less, morepreferably 4.0 mass % or more and 12 mass % or less, still morepreferably 5.0 mass % or more and 10 mass % or less.

From the viewpoint of improving wettability to the pigment and theadsorbability of the water-insoluble polymer to the pigment, the contentof the organic solvent in the pigment mixture is preferably 10 mass % ormore and 35 mass % or less, more preferably 12 mass % or more and 30mass % or less, still more preferably 15 mass % or more and 25 mass % orless.

From the viewpoint of improving the dispersion stability of the pigmentaqueous dispersion and the viewpoint of improving the productivity ofthe pigment aqueous dispersion, the content of the water in the pigmentmixture is preferably 40 mass % or more and 75 mass % or less, morepreferably 45 mass % or more and 70 mass % or less, still morepreferably 50 mass % or more and 65 mass % or less.

From the viewpoint of improving the dispersion stability of the pigmentaqueous dispersion and the storage stability of the ink for gravureprinting, the mass ratio “[pigment/water-insoluble polymer]” of thepigment to the water-insoluble polymer a is preferably from 30/70 to90/10, more preferably from 40/60 to 85/15, still more preferably from50/50 to 75/25.

In the step I, further, the pigment mixture is dispersed to obtain adispersion treatment product. A dispersion method for obtaining thedispersion treatment product is not particularly limited. The pigmentparticles may be atomized until their average particle diameter becomesa desired particle diameter by only main dispersion, but it is preferredthat the pigment mixture be subjected to preliminary dispersion, andthen be further subjected to the main dispersion by applying a shearstress so as to control the average particle diameter of the pigmentparticles to the desired particle diameter.

A temperature in the preliminary dispersion of the step I is preferably0° C. or more, and is preferably 40° C. or less, more preferably 30° C.or less, still more preferably 20° C. or less, and a dispersion time ispreferably 0.5 hour or more and 30 hours or less, more preferably 1 houror more and 20 hours or less, still more preferably 1 hour or more and10 hours or less.

In the preliminary dispersion of the pigment mixture, a generally usedmixing and stirring apparatus, such as an anchor blade or a disperblade, may be used. Of those, a high-speed stirring and mixing apparatusis preferred.

As means for applying a shear stress in the main dispersion, there aregiven, for example: kneading machines, such as a roll mill and akneader; high-pressure homogenizers, such as Microfluidizer(manufactured by Microfluidics Corp.); and medium-type dispersingmachines, such as a paint shaker and a bead mill. Commercially availableexamples of the medium-type dispersing machines include Ultra Apex Mill(manufactured by Kotobuki Industries Co., Ltd.) and PICO MILL(manufactured by Asada Iron Works Co., Ltd.). Those apparatus may beused in combination thereof. Of those, from the viewpoint of reducingthe particle diameter of the pigment, a high-pressure homogenizer ispreferably used.

When the main dispersion is performed using the high-pressurehomogenizer, the pigment may be controlled so as to have a desiredparticle diameter through the control of a treatment pressure and thenumber of passes.

From the viewpoints of productivity and economic efficiency, thetreatment pressure is preferably 60 MPa or more and 200 MPa or less,more preferably 100 MPa or more and 180 MPa or less, still morepreferably 130 MPa or more and 180 MPa or less.

In addition, the number of passes is preferably 3 or more and 30 orless, more preferably 5 or more and 25 or less.

(Step II)

In the step II, the organic solvent is removed from the obtaineddispersion by a known method, and thus an aqueous dispersion of thepigment-containing polymer particles can be obtained. It is preferredthat the organic solvent in the obtained aqueous dispersion containingthe pigment-containing polymer particles be substantially removed, butthe organic solvent may remain as long as the object of the presentinvention is not impaired. The amount of the residual organic solvent ispreferably 0.1 mass % or less, more preferably 0.01 mass % or less.

In addition, as necessary, the dispersion may be subjected to heatingand stirring treatment before the evaporation of the organic solvent.

The obtained aqueous dispersion of the pigment-containing polymerparticles is such that solid water-insoluble polymer particles eachcontaining the pigment are dispersed in media in which water serves as amain medium. In this case, the form of the water-insoluble polymerparticles is not particularly limited, and it is only necessary that theparticles be each formed of at least the pigment and the water-insolublepolymer. Examples thereof include: a particle form in which the pigmentis encapsulated in the water-insoluble polymer; a particle form in whichthe pigment is uniformly dispersed in the water-insoluble polymer; aparticle form in which the pigment is exposed on the surface of each ofthe water-insoluble polymer particles; and a mixture thereof.

(Step III)

The step III, which is an optional step, is a step of performingcross-linking treatment by mixing the aqueous dispersion obtained in thestep II and a cross-linking agent, to thereby obtain an aqueousdispersion.

In this case, when the water-insoluble polymer is an anionicwater-insoluble polymer having an anionic group, the cross-linking agentis preferably a compound having a functional group capable of reactingwith the anionic group, more preferably a compound having 2 or more,preferably 2 to 6 of the functional groups in the molecule.

Suitable examples of the cross-linking agent include a compound having 2or more epoxy groups in the molecule, a compound having 2 or moreoxazoline groups in the molecule, and a compound having 2 or moreisocyanate groups in the molecule. Of those, a compound having or moreepoxy groups in the molecule is preferred, and trimethylolpropanepolyglycidyl ether is more preferred.

From the viewpoint of improving the dispersion stability of the pigmentaqueous dispersion and the viewpoint of facilitating the preparation ofthe ink for gravure printing, the non-volatile component concentration(solid content concentration) of the obtained pigment aqueous dispersionis preferably 10 mass % or more and 30 mass % or less, more preferably15 mass % or more and 25 mass % or less.

From the viewpoint of reducing coarse particles, the average particlediameter of the pigment-containing polymer particles in the pigmentaqueous dispersion is preferably 30 nm or more and 200 nm or less, morepreferably 40 nm or more and 180 nm or less, still more preferably 50 nmor more and 170 nm or less.

The average particle diameter of the pigment-containing polymerparticles is measured by a method to be described later in Examples.

In addition, the average particle diameter of the pigment-containingpolymer particles in the aqueous ink is the same as the average particlediameter in the pigment aqueous dispersion, and a preferred mode of theaverage particle diameter is the same as the preferred mode of theaverage particle diameter in the pigment aqueous dispersion.

(Content of Each Component of Pigment-Containing Polymer Particles inAqueous Ink)

From the viewpoint of printing density, the content of the pigment inthe ink is preferably 1 mass % or more and 20 mass % or less, morepreferably 2 mass % or more and 18 mass % or less, still more preferably3 mass % or more and 15 mass % or less.

From the viewpoints of printing density and fixability, the content ofthe pigment-containing polymer particles in the ink is preferably 1 mass% or more and 30 mass % or less, more preferably 3 mass % or more and 25mass % or less, still more preferably 5 mass % or more and 20 mass % orless.

From the viewpoint of fixability, the content of the water-insolublepolymer in the pigment-containing polymer particles in the ink ispreferably 1 mass % or more and 20 mass % or less, more preferably 2mass % or more and 15 mass % or less, still more preferably 3 mass % ormore and 10 mass % or less.

[Water-Insoluble Polymer b]

The water-insoluble polymer b is formed of polymer particles free of thepigment. As a component thereof, there are given, for example, anacrylic resin, a styrene-based resin, a urethane-based resin, apolyester-based resin, a styrene-acrylic resin, a butadiene-based resin,a styrene-butadiene-based resin, a vinyl chloride-based resin, a vinylacetate-based resin, and an acrylic silicone-based resin. Of those, anacrylic resin is preferred from the viewpoint of accelerating a dryingproperty on a printing substrate to improve the rub fastness of aprinted product.

In addition, from the viewpoint of improving the productivity of thewater-based ink, the water-insoluble polymer b is preferably used as adispersion liquid containing water-insoluble polymer particles. As thewater-insoluble polymer particles, appropriately synthesized ones may beused, or a commercially available product may be used.

The water-insoluble polymer b is produced by copolymerizing a mixture ofmonomers by a known polymerization method. Preferred examples of thepolymerization method include an emulsion polymerization method and asuspension polymerization method. Of those, an emulsion polymerizationmethod is more preferred.

A polymerization initiator may be used in the polymerization. Examplesof the polymerization initiator include a persulfate and a water-solubleazo polymerization initiator. Of those, a persulfate, such as ammoniumpersulfate or potassium persulfate, is preferred.

A surfactant may be used in the polymerization. Examples of thesurfactant include a nonionic surfactant, an anionic surfactant, and acationic surfactant. Of those, a nonionic surfactant is preferred fromthe viewpoint of improving the dispersion stability of the resinparticles. Examples of the nonionic surfactant include a polyoxyethylenealkyl ether, a polyoxyethylene alkyl aryl ether, a polyoxyethylene fattyacid ester, and an oxyethylene/oxypropylene block copolymer. Of those, apolyoxyethylene alkyl ether is preferred from the viewpoint of improvingthe dispersion stability of the resin particles.

Preferred polymerization conditions vary depending on, for example, thekind of the polymerization initiator, but a polymerization temperatureis preferably 50° C. or more and 90° C. or less, and a polymerizationtime is preferably 1 hour or more and 20 hours or less. In addition, apolymerization atmosphere is preferably a nitrogen gas atmosphere or aninert gas atmosphere such as argon.

After the completion of the polymerization reaction, the generatedpolymer may be isolated from the reaction solution by a known method,for example, reprecipitation or solvent evaporation. In addition, anunreacted monomer and the like may be removed from the obtained polymerby reprecipitation, membrane separation, chromatography, an extractionmethod, or the like.

From the viewpoint of compatibility in the ink, the water-insolublepolymer b is preferably used as a polymer dispersion using water as adispersion medium without the removal of the solvent used in thepolymerization reaction.

Commercially available examples of the dispersion of the water-insolublepolymer b include: acrylic resins, such as “Neocryl A1127” (manufacturedby DSM NeoResins, Inc., anionic self-cross-linking water-based acrylicresin) and “Joncryl 390” (manufactured by BASF Japan Ltd.); urethaneresins, such as “WBR-2018” and “WBR-2000U” (manufactured by Taisei FineChemical Co., Ltd.); styrene-butadiene resins, such as “SR-100” and“SR102” (both of which are manufactured by Nippon A & L Inc.);styrene-acrylic resins, such as “Joncryl 7100”, “Joncryl 734”, and“Joncryl 538” (all of which are manufactured by BASF Japan Ltd.); andvinyl chloride-based resins, such as “Vinyblan 701” (manufactured byNissin Chemical Industry Co., Ltd.).

As a form of the water-insoluble polymer b, there are given particlesdispersed in water. The dispersion of the water-insoluble polymerparticles forms a film on a printing substrate to improve fixability.

From the viewpoint of the fixability of the ink, the content of thewater-insoluble polymer b in the ink is preferably 1 mass % or more and30 mass % or less, more preferably 2 mass % or more and 20 mass % orless, still more preferably 3 mass % or more and 15 mass % or less. Whenthe content is less than the lower limit of the above-mentioned range,the fixability of the ink is lowered, and when the content is more thanthe upper limit, the storage stability of the ink may be lowered.

From the viewpoint of the fixability, the weight-average molecularweight of the water-insoluble polymer b to be used in the presentinvention is preferably 100,000 or more, more preferably 200,000 ormore, still more preferably 500,000 or more, and is preferably 2,500,000or less, more preferably 1,000,000 or less.

In addition, from the viewpoint of the storage stability of the ink, theaverage particle diameter of the water-insoluble polymer particles inthe dispersion or ink containing the water-insoluble polymer particlesis preferably 10 nm or more, more preferably 30 nm or more, still morepreferably 50 nm or more, and is preferably 300 nm or less, morepreferably 200 nm or less, still more preferably 150 nm or less, evenstill more preferably 130 nm or less.

From the viewpoint of the storage stability of the ink, the acid valueof the water-insoluble polymer b to be used in the present invention ispreferably 1 or more and 45 or less, more preferably 3 or more and 40 orless, still more preferably 5 or more and 35 or less.

The weight-average molecular weight and the average particle diameter ofthe water-insoluble polymer b are measured by methods described inExamples.

From the viewpoint of the stability of the ink, the ratio between thepigment and the water-insoluble polymers (total amount of the polymer aand the polymer b) in the ink to be used in the present invention ispreferably from 100/20 to 100/300, more preferably from 100/30 to100/280, still more preferably from 100/50 to 100/250.

[Water-Soluble Organic Solvent]

The aqueous ink to be used in the present invention preferably containsa water-soluble organic solvent from the viewpoint of adjusting theviscosity of the ink and its transfer amount onto the printing medium.

The water-soluble organic solvent to be used in the present inventionmay be freely used irrespective of whether the water-soluble organicsolvent is a liquid or a solid at normal temperature. The water-solubleorganic solvent has a boiling point of preferably 100° C. or more and260° C. or less, more preferably 110° C. or more and 250° C. or less,still more preferably 120° C. or more and 240° C. or less. When theboiling point is less than the lower limit of the above-mentioned range,the highlight suitability is lowered, and when the boiling point is morethan the upper limit, the drying property may be lowered.

The molecular weight of the water-soluble organic solvent is preferably60 or more and 200 or less, more preferably 80 or more and 190 or less,still more preferably 100 or more and 180 or less.

Such water-soluble organic solvent is preferably a glycol or a glycolether, more preferably a glycol ether. Those water-soluble organicsolvents may be used in combination thereof.

Examples of the glycol include propylene glycol (188° C.),1,2-butanediol (194° C.), ethylene glycol (197° C.),3-methyl-1,3-butanediol (203° C.), 1,2-pentanediol (210° C.),2-methyl-1,3-propanediol (214° C.), 1,2-hexanediol (224° C.),1,3-propanediol (230° C.), dipropylene glycol (231° C.), and diethyleneglycol (244° C.). Numerical values in parentheses represent boilingpoints. Those glycols may be used in combination thereof. The glycolpreferably contains propylene glycol from the viewpoints of the dryingproperty and highlight suitability of the ink.

Examples of the glycol ether include ethylene glycol monomethyl ether(125° C.), ethylene glycol monoisopropyl ether (142° C.), ethyleneglycol monobutyl ether (171° C.), ethylene glycol monoisobutyl ether(161° C.), diethylene glycol monomethyl ether (194° C.), diethyleneglycol monoisopropyl ether (207° C.), diethylene glycol monobutyl ether(231° C.), diethylene glycol monoisobutyl ether (220° C.), propyleneglycol monomethyl ether (121° C.), propylene glycol monopropyl ether(150° C.), dipropylene glycol monomethyl ether (187° C.), tripropyleneglycol monomethyl ether (220° C.), diethylene glycol dimethyl ether(162° C.), diethylene glycol methyl ethyl ether (176° C.), diethyleneglycol diethyl ether (189° C.), and triethylene glycol dimethyl ether(216° C.). Numerical values in the parentheses represent boiling points.Those glycol ethers may be used in combination thereof. Of those glycolethers, diethylene glycol monomethyl ether, diethylene glycolmonoisopropyl ether, diethylene glycol monobutyl ether, diethyleneglycol monoisobutyl ether, dipropylene glycol monomethyl ether,tripropylene glycol monomethyl ether, diethylene glycol dimethyl ether,diethylene glycol methyl ethyl ether, and diethylene glycol diethylether are preferred from the viewpoints of the drying property andhighlight suitability of the ink.

From the viewpoint of improving the highlight suitability and the dryingproperty, the content of the water-soluble organic solvent in the ink ispreferably 10 mass % or more and 35 mass % or less, more preferably 12mass % or more and 32 mass % or less, still more preferably 13 mass % ormore and 30 mass % or less.

[Surfactant]

The ink may contain a surfactant. The surfactant is preferably selectedfrom an anionic surfactant, a nonionic surfactant, and an amphotericsurfactant, and those surfactants may be used in combination thereof.

Of those, from the viewpoint of improving the storability of thedispersion liquid, a nonionic surfactant is particularly preferred.Examples of the nonionic surfactant include alcohol-based, acetyleneglycol-based, silicone-based, and fluorine-based surfactants, and thosenonionic surfactants may be used in combination thereof. From theviewpoint of wettability to a printing substrate, a silicone-basedsurfactant is particularly preferred.

From the viewpoint of the wettability to a printing substrate, thealcohol-based surfactant is preferably an alkylene oxide adduct of analcohol having 6 or more and 30 or less carbon atoms.

From the same viewpoint as above, the number of carbon atoms of thealcohol is preferably 8 or more, more preferably 10 or more, still morepreferably 12 or more, and is preferably 24 or less, more preferably 22or less, still more preferably 20 or less.

From the same viewpoint as above, the alkylene oxide adduct ispreferably an ethylene oxide adduct or an ethylene oxide and propyleneoxide adduct, more preferably an ethylene oxide adduct.

As commercially available products of the alcohol-based surfactant,there are given, as an ethylene oxide adduct of lauryl alcohol, EMULGEN108 (HLB: 12.1, average number of moles of EO added: 6), EMULGEN 109P(HLB: 13.6, average number of moles of EO added: 8), EMULGEN 120 (HLB:15.3, average number of moles of EO added: 13), EMULGEN 147 (HLB: 16.3,average number of moles of EO added: 17), and EMULGEN 150 (HLB: 18.4,average number of moles of EO added: 44) manufactured by KaoCorporation. Besides, there are given, for example, EMULGEN 707(ethylene oxide adduct of a secondary alcohol having 11 to 15 carbonatoms, HLB: 12.1, average number of moles of EO added: 6) and EMULGEN220 (ethylene oxide adduct of a linear primary alcohol having 16 to 18carbon atoms, HLB: 14.2, average number of moles of EO added: 13)manufactured by Kao Corporation.

From the viewpoint of the wettability to a printing substrate, examplesof the acetylene glycol-based surfactant include one or more kinds ofacetylene glycols selected from 2,4,7,9-tetramethyl-5-decyne-4,7-diol,3,6-dimethyl-4-octyne-3,6-diol, and 2,5-dimethyl-3-hexyne-2,5-diol, andethylene oxide adducts of these acetylene glycols.

As commercially available products thereof, there are given, forexample: Surfynol 104 (2,4,7,9-tetramethyl-5-decyne-4,7-diol, averagenumber of moles of EO added: 0, HLB: 3.0), Surfynol 104E (50% dilutedproduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol with ethylene glycol),Surfynol 104PG-50 (50% diluted product of2,4,7,9-tetramethyl-5-decyne-4,7-diol with propylene glycol), andSurfynol 420 (EO adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol havingan average number of moles of EO added of 1.3, HLB: 4.7) manufactured byNissin Chemical Industry Co., Ltd. and Air Products & Chemicals, Inc.;and ACETYLENOL E13T (average number of moles of EO added: 1.3, HLB: 4.7)manufactured by Kawaken Fine Chemicals Co., Ltd.

Examples of the silicone-based surfactant include dimethylpolysiloxane,a polyether-modified silicone, an amino-modified silicone, acarboxy-modified silicone, methyl phenyl polysiloxane, a fattyacid-modified silicone, an alcohol-modified silicone, an aliphaticalcohol-modified silicone, an epoxy-modified silicone, afluorine-modified silicone, a cyclic silicone, and an alkyl-modifiedsilicone. Of those, a polyether-modified silicone is preferred from theviewpoint of the wettability to a printing substrate.

Examples of the polyether-modified silicone include PEG-3 dimethicone,PEG-9 dimethicone, PEG-9PEG-9 dimethicone, PEG-9 methyl etherdimethicone, PEG-10 dimethicone, PEG-11 methyl ether dimethicone,PEG/PPG-20/22 butyl ether dimethicone, PEG-32 methyl ether dimethicone,PEG-9 polydimethylsiloxyethyl dimethicone, and lauryl PEG-9polydimethylsiloxyethyl dimethicone. Of those, PEG-11 methyl etherdimethicone is particularly preferred.

As commercially available products thereof, there are given, forexample, Silicone KF-6011, KF-6012, KF-6013, KF-6015, KF-6016, KF-6017,KF-6028, KF-6038, and KF-6043 manufactured by Shin-Etsu Chemical Co.,Ltd.

From the viewpoint of improving the wettability to a printing substrate,the content of the surfactant in the ink is preferably 0.01 mass % ormore and 5.0 mass % or less, more preferably 0.1 mass % or more and 2.0mass % or less, still more preferably 0.2 mass % or more and 1.0 mass %or less.

[Water]

From the viewpoints of the drying property and a VOC reduction, thecontent of the water in the ink is preferably 50 mass % or more and 70mass % or less, more preferably 52 mass % or more and 68 mass % or less,still more preferably 55 mass % or more and 65 mass % or less. When theink contains a component other than the pigment, the polymer, thewater-soluble organic solvent, the surfactant, and the water, the othercomponent may be contained to replace part of the content of the water.

[Optional Components of Ink for Gravure Printing]

To the ink, various additives, such as a pH adjuster, a viscositymodifier, an antifoaming agent, a preservative, a fungicide, and acorrosion inhibitor, may be further added.

[Printing Medium]

Examples of the printing medium to be used for printing in the presentinvention include: papers, such as coated paper, art paper, syntheticpaper, and processed paper; and resin films, such as a polyester film, apolyethylene film, a polypropylene film, a polystyrene film, a vinylchloride film, and a nylon film. The printing medium is preferably aresin film from the viewpoint of an increase in printing density. Ofthose, the printing medium is preferably a polyester film or apolypropylene film from the viewpoint of post-processing suitability.From the viewpoint of improving gravure printing suitability, a filmsubjected to surface treatment by discharge processing, such as coronatreatment or plasma treatment, may be used.

EXAMPLES

Now, the present invention is more specifically described by way ofExamples and the like. In the following Examples and the like, physicalproperties were measured by the following methods. “Part(s)” and “%”mean “part(s) by mass” and “mass %” unless otherwise specified.

(1) Measurement of Weight-Average Molecular Weight of Polymer

With the use of a liquid obtained by dissolving, in N,N-dimethylformamide, phosphoric acid and lithium bromide atconcentrations of 60 mmol/L and 50 mmol/L, respectively, as an eluent,the molecular weight of a water-insoluble polymer was measured by a gelpermeation chromatography method [GPA apparatus manufactured by TosohCorporation (HLA-8120GPA), columns manufactured by Tosoh Corporation(two TSK-GEL, α-M columns), flow rate: 1 mL/min]. Monodispersedpolystyrene having a known molecular weight was used as a referencematerial.

(2) Measurement of Average Particle Diameter of Particles

Measurement was performed using a laser particle analysis system(manufactured by Otsuka Electronics Co., Ltd., model number: ELS-8000,cumulant analysis). A dispersion liquid diluted with water so that theconcentration of particles to be measured was about 5×10⁻³ mass % wasused. Measurement conditions were a temperature of 25° C., an anglebetween incident light and a detector of 90°, and a cumulative number of100 times, and the refractive index (1.333) of water was input as therefractive index of a dispersion solvent.

(3) Measurement of Acid Value

In a potentiometric automatic titration apparatus (manufactured by KyotoElectronics Manufacturing Co., Ltd., electric burette, model number:APB-610), a polymer is dissolved in a titration solvent obtained bymixing toluene and acetone (2+1) and the solution is titrated with a 0.1N potassium hydroxide/ethanol solution by a potentiometric titrationmethod. An inflection point on a titration curve is defined as an endpoint. An acid value was calculated from the titre of the potassiumhydroxide solution required to reach the end point.

Production Example I (Production of Water-Insoluble Polymer a Solution)

The monomer, solvent, polymerization initiator(2,2′-azobis(2,4-dimethylvaleronitrile) (manufactured by Wako PureChemical Industries, Ltd., product name: V-65), and polymerization chaintransfer agent (2-mercaptoethanol) (manufactured by Kishida ChemicalCo., Ltd.) of kinds shown in the “initially loaded monomer solution”column of Table 1 were placed in a reaction vessel with two droppingfunnels 1 and 2, and were mixed, followed by nitrogen gas purging. Thus,an initially loaded monomer solution was obtained.

Next, the monomers, solvent, polymerization initiator, andpolymerization chain transfer agent shown in the “monomer solution 1 tobe dropped” column of Table 1 were mixed to provide a monomer solution 1to be dropped, which was placed in the dropping funnel 1, followed bynitrogen gas purging. In addition, the monomers, solvent, polymerizationinitiator, and polymerization chain transfer agent shown in the “monomersolution 2 to be dropped” column of Table 1 were mixed to provide amonomer solution 2 to be dropped, which was placed in the droppingfunnel 2, followed by nitrogen gas purging.

The macromonomer in Table 1 is a 50 mass % toluene solution of a productavailable from Toagosei Co., Ltd. under the product name “AS-6S”, whichhas a number-average molecular weight of 6,000. NK Ester TM-40G is theproduct name of methoxypolyethylene glycol monomethacrylate manufacturedby Shin-Nakamura Chemical Co., Ltd. (average number of moles of ethyleneoxide added=4). BLEMMER PP1000 is the product name of polypropyleneglycol monomethacrylate manufactured by NOF Corporation (average numberof moles of propylene oxide added=5, terminal: hydrogen atom). Thepolymerization initiator V-65 is the product name of2,2′-azobis(2,4-dimethylvaleronitrile) manufactured by Wako PureChemical Industries, Ltd. In addition, for methacrylic acid and styrenein Table 1, reagents manufactured by Wako Pure Chemical Industries, Ltd.were used.

Under a nitrogen atmosphere, the initially loaded monomer solution inthe reaction vessel was kept at 77° C. while being stirred, and themonomer solution 1 to be dropped in the dropping funnel 1 was slowlydropped into the reaction vessel over 3 hours. Then, the monomersolution 2 to be dropped in the dropping funnel 2 was slowly droppedinto the reaction vessel over 2 hours. After the completion of thedropping, the mixed solution in the reaction vessel was stirred at 77°C. for 0.5 hour.

Then, a polymerization initiator solution of 1 part of theabove-mentioned polymerization initiator (V-65) dissolved in 100 partsof methyl ethyl ketone (manufactured by Wako Pure Chemical Industries,Ltd.) was prepared and added to the mixed solution, and the resultantwas stirred at 77° C. for 0.5 hour to perform aging. The preparation andaddition of the polymerization initiator solution, and aging describedabove were further performed five times. Then, the reaction solution inthe reaction vessel was kept at 80° C. for 1 hour, and 429 parts ofmethyl ethyl ketone was added so as to achieve a solid contentconcentration of 38.0%. Thus, a water-insoluble polymer a solution wasobtained. The water-insoluble polymer a had a weight-average molecularweight of 62,000 and an acid value of 102.

TABLE 1 Initially Monomer Monomer loaded solution 1 solution 2 Monomermonomer to be to be loading solution dropped dropped ratio (part(s))(part(s)) (part(s)) (%) Styrene 39.6 316.8 39.6 44 Macromonomer AS-6S27.0 243.0 0.0 15 Methacrylic acid 0.0 115.2 28.8 16 NK Ester TM-40G22.1 176.4 22.1 24.5 BLEMMER PP-1000 0.5 3.6 0.5 0.5 MEK 15.8 173.3126.0 Mercaptoethanol 0.1 0.9 0.3 V-65 0.0 7.2 1.8

In Table 1, the blending amount of each blended material is expressed inpart(s) by mass. The monomer loading ratio in Table 1 is a value at asolid content of 50% in the case of the macromonomer, and is a value ata solid content of 100% for each of the other monomers. In addition, theloading amounts of the monomer solutions in Table 1 are values insolutions.

Production Example II (Production of Dispersion Liquid ofWater-Insoluble Polymer b Particles)

0.5 g of methacrylic acid, 14.5 g of methyl methacrylate (manufacturedby Wako Pure Chemical Industries, Ltd.), 5.0 g of 2-ethylhexyl acrylate(manufactured by Wako Pure Chemical Industries, Ltd.), LATEMUL E-118B(11.1 g of sodium polyoxyethylene alkyl ether sulfate, manufactured byKao Corporation, surfactant), 0.2 g of potassium persulfate serving as apolymerization initiator (manufactured by Wako Pure Chemical Industries,Ltd.), and 282.8 g of ion-exchanged water were placed in a reactionvessel with a dropping funnel, and were mixed at 150 rpm, followed bynitrogen gas purging. Thus, an initially loaded monomer solution wasobtained.

A monomer solution to be dropped obtained by mixing 9.5 g of methacrylicacid, 275.5 g of methyl methacrylate, 95.0 g of 2-ethylhexyl acrylate,35.1 g of LATEMUL E-118B, 0.6 g of potassium persulfate, and 183.0 g ofion-exchanged water at 150 rpm was placed in the dropping funnel, andnitrogen gas purging was performed.

Under a nitrogen atmosphere, the initially loaded monomer solution inthe reaction vessel was increased in temperature from room temperatureto 80° C. over 30 minutes while being stirred at 150 rpm. While thetemperature was kept at 80° C., the monomers in the dropping funnel wereslowly dropped into the reaction vessel over 3 hours. After thecompletion of the dropping, while the temperature in the reaction vesselwas kept, the contents were stirred for 1 hour, and 204.7 parts ofion-exchanged water was added. Then, the resultant was filtered througha stainless-steel wire mesh (200 mesh) to provide a dispersion liquid ofwater-insoluble polymer b particles (solid content concentration: 40%,average particle diameter: 100 nm, acid value: 16, Tg: 48° C.).

Production Example III (Production of Aqueous Dispersion a ofPigment-Containing Polymer Particles)

225.6 Parts of the water-insoluble polymer a solution (solid contentconcentration: 38.0%) obtained in Production Example I was mixed with72.6 parts of methyl ethyl ketone (MEK) to provide a MEK solution of thewater-insoluble polymer a. The MEK solution of the water-insolublepolymer a was put into a vessel having a disper blade and having avolume of 2 L. While the solution was stirred under the condition of1,400 rpm, 681.9 parts of ion-exchanged water, 29.8 parts of a 5 Nsodium hydroxide aqueous solution (manufactured by Wako Pure ChemicalIndustries, Ltd.), and 2.3 parts of a 25% ammonia aqueous solution(manufactured by Wako Pure Chemical Industries, Ltd.) were added, andthe degree of neutralization with sodium hydroxide and the degree ofneutralization with ammonia were adjusted to 78.8 mol % and 21.2 mol %,respectively. The resultant was stirred at 1,400 rpm for 15 minuteswhile being cooled in a water bath at 0° C.

Then, 200 parts of carbon black (manufactured by Cabot Japan K.K.,product name: MONARCH 717) was added, and the mixture was stirred at6,400 rpm for 1 hour. The resultant pigment mixture was subjected to9-pass dispersion treatment at a pressure of 150 MPa usingMicrofluidizer “M-110EH” (manufactured by Microfluidics Corp.) toprovide a dispersion treatment product (having a solid contentconcentration of 20%).

600 Parts of the dispersion treatment product obtained in theabove-mentioned step was placed in a 2 L recovery flask, and 200 partsof ion-exchanged water was added (solid content concentration: 15.0%).The mixture was kept at a pressure of 0.09 MPa for 3 hours in a warmbath adjusted to 32° C. using a rotary evaporator (N-1000S, manufacturedby Tokyo Rikakikai Co., Ltd.) at a number of rotations of 50 r/min, tothereby remove the organic solvent. Further, the resultant wasconcentrated to a solid content concentration of 25% by adjusting thewarm bath to 62° C. and reducing the pressure to 0.07 MPa.

The resultant concentrate was put into a 500 ml angle rotor, andcentrifuged at 7,000 rpm for 20 minutes using a high-speed refrigeratedcentrifuge (himaa AR22G, manufactured by Hitachi Koki Co., Ltd., presettemperature: 20° C.). After that, the liquid layer portion was filteredthrough a 5 μm membrane filter (manufactured by Sartorius, MinisartMAP-010XS).

To 400 parts of the filtrate obtained in the foregoing (pigment: 68.6parts, water-insoluble polymer a: 29.4 parts), 2.1 parts of DenacolEX-321L (manufactured by Nagase ChemteX Corporation, trimethylolpropanepolyglycidyl ether, epoxy equivalent: 129) (corresponding to 25 mol %with respect to a carboxylic acid serving as a cross-linking reactionpoint contained in methacrylic acid in the polymer) and 0.91 part ofPROXEL LV(S) (manufactured by Lonza Japan Ltd, antifungal agent, activecomponent: 20%) were added, and 51.94 parts of ion-exchanged water wasfurther added so as to achieve a solid content concentration of 22.0%.The resultant was stirred at 70° C. for 3 hours, and then filteredthrough a 5 μm membrane filter (manufactured by Sartorius, MinisartMAP-010XS) to provide an aqueous dispersion A containing 22% ofpigment-containing polymer particles (pigment aqueous dispersion;average particle diameter: 105 nm).

Preparation of Ink for Gravure Printing Production Example 1 (Productionof Ink 1)

In order to achieve an ink composition shown in Table 2, in a productioncontainer, to 65.4 parts of the aqueous dispersion A described inProduction Example III (corresponding to a pigment concentration in inkof 10%, solid content concentration: 22%), 0.59 part of a neutralizer(manufactured by Wako Pure Chemical Industries, Ltd., 1 N sodiumhydroxide solution) and 8.25 parts of the aqueous dispersion liquid bdescribed in Production Example II (corresponding to a polymerconcentration in ink of 3.3%, solid content concentration: 40%) wereadded, and the mixture was stirred at 150 rpm. Further, 5 parts ofpropylene glycol (manufactured by Wako Pure Chemical Industries, Ltd.),20 parts of diethylene glycol monoisobutyl ether, 0.5 part of asurfactant (manufactured by Shin-Etsu Chemical Co., Ltd., product name:KF-6011, PEG-11 methyl ether dimethicone), and 0.26 part ofion-exchanged water were added, and the mixture was stirred under roomtemperature for 30 minutes and then filtered through a stainless-steelwire mesh (200 mesh) to provide an ink 1.

TABLE 2 Com- Com- parative parative Example 1 Example 2 Example 3Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 1Example 2 Gravure plate 175 lines 250 lines 250 lines 250 lines numberCell depth (μm) 6 8 10 5 6 6 6 Cell volume 4.2 5.7 6.5 3.2 4.0 4.0 4.0(ml/m²) Ink number 1 1 1 1 1 2 3 4 5 Pigment 10 10 10 10 10 10 10 10 10Polymer a 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 Polymer b 3.3 3.3 3.3 3.33.3 3.3 3.3 3.3 3.3 PG 5 5 5 5 5 5 BG 20 MDG 20 iBDG 20 20 20 20 20 8BDG 20 Surfactant 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Neutralizer 0.020.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 Ion-exchanged 56.68 56.68 56.6856.68 56.68 61.68 61.68 61.68 68.68 water Zahn cup #3 17.2 12.5 11.714.4 14.4 17.8 13.3 (seconds) Ink evaporation 14 26 20 12 22 33 56 rate(%) Ink transfer 2.7 4.5 5.4 2.3 3.0 2.4 2.5 2.5 2.5 1.8 1.4 amount(ml/m²) Ink transfer 64 79 83 72 75 60 62 62 63 44 36 ratio (%) Printingdensity 1.80 2.31 2.40 2.02 2.05 1.60 1.68 1.67 1.70 0.80 1.02 Halftonedot A A A A A B A A B C C area ratio

In Table 2, the blending amount of each blended material is expressed inpart(s) by mass. Symbols in Table 2 represent the following.

PG: propylene glycol (manufactured by Wako Pure Chemical Industries,Ltd., boiling point: 188° C.)BG: ethylene glycol monobutyl ether (manufactured by Wako Pure ChemicalIndustries, Ltd., boiling point: 171° C.)MDG: diethylene glycol monomethyl ether (manufactured by Wako PureChemical Industries, Ltd., boiling point: 194° C.)iBDG: diethylene glycol monoisobutyl ether (manufactured by Wako PureChemical Industries, Ltd., boiling point: 220° C.)BDG: diethylene glycol monobutyl ether (manufactured by Wako PureChemical Industries, Ltd., boiling point: 231° C.)

The obtained ink was measured for its viscosity and evaporation rate bythe following evaluation methods. The results are shown in Table 2.

<Evaluation Method for Zahn Cup #3 Viscosity>

The ink was immersed in a thermostatic bath having a water temperatureadjusted to 20° C. for 30 minutes or more, the efflux time of the inkwas measured using a Zahn cup #3 (manufactured by Meisei Corporation),and the measured value was adopted as a Zahn cup viscosity.

<Evaluation Method for Ink Evaporation Rate>

1 g of the ink was put into a dryer adjusted to 40° C., and was dried atan air flow of 1,400 L/min for 30 minutes. After that, an inkevaporation rate was determined by the following equation (1). As thedryer, a dryer (manufactured by Yamato Scientific Co., Ltd., modelnumber: Vacuum Oven DP33) having a meter for air management(manufactured by Azbil Corporation, model number: MCF0250) mounted onits air intake side was used.

Ink evaporation rate (%)=[1−(mass of ink after drying/mass of ink beforedrying)]×100  (1)

Production Examples 2 to 5 (Production of Inks 2 to 5)

Inks 2 to 5 were each obtained in accordance with the production methoddescribed in Production Example 1 except that changes were made so as toachieve an ink composition shown in Table 2. The obtained inks were eachmeasured for its viscosity and evaporation rate by the same methods asthose of Production Example 1. The results are shown in Table 2.

Example 1 <Printing Test>

With the use of the ink 1 of Production Example 1, printing wasperformed on the corona-treated surface of an OPP film (manufactured byFutamura Chemical Co., Ltd., FOR-AQ#20, Laminate grade). In theprinting, a halftone dot pattern (1%, 3%, 5%, 10%, 20%, 30%, 40%, 50%,60%, 70%, 80%, 90%, or 100%) was printed with a gravure printing machine(eight-color press manufactured by Orient Sogyo Co., Ltd., OSG-SDX TypeVLS) having mounted thereon a gravure roll (manufactured by ThinkLaboratory Co., Ltd., laser plate making system, gravure plate: 175lines/inch, cell shape: gravure dot (quadrilateral shape), gravure celldepth: 6 μm) under a printing condition (printing speed: 30 m/min),followed by warm-air drying.

<Evaluation Method for Gravure Cell Volume>

With regard to the volume of each gravure cell, a roll inspectionapparatus (confocal laser microscope manufactured by Think LaboratoryCo., Ltd., objective lens: 50×) was placed on the roll, and scanning wasperformed from the bottom surface of each cell in a 100% halftone dotprinted portion to the top surface thereof to measure a volume per cell,which was converted to a gravure cell volume per 100 mm×100 mm by usingthe number of lines at which plate making had been performed as thedensity of the cells.

<Evaluation Method for Ink Transfer Amount>

A measurement method for the transfer amount of an ink is as describedbelow. Ten 100 mm×100 mm square printed products of a 100% halftone dotprinted portion are prepared. First, the initial weight of each of theprinted products is measured, and then the ink coating of each of allthe ten printed products is dissolved and removed with an ink-dilutingsolvent (Daireducer manufactured by DIC Corporation). After that, thesubstrates are subjected to warm-air drying at 45° C. for 24 hours, andare each measured for its weight. A difference between the initialweight and the weight after the ink removal and the drying was adoptedas an ink transfer amount. The ink transfer amount is the average valueof the ten printed products.

<Evaluation Method for Ink Transfer Ratio>

The transfer ratio of an ink is a value determined by the followingequation (2) from the gravure cell volume and the ink transfer amountdetermined as described above.

Ink transfer ratio (%)=(ink transfer amount/gravure cellvolume)×100  (2)

<Evaluation Method for Printing Density>

A 100% halftone dot printed portion was subjected to measurement using aspectrophotometer (manufactured by GretagMacbeth, product name:SpectroEye) in a density measurement mode (DIN, Abs). The results areshown in Table 2.

<Evaluation Method for Highlight Suitability>

The halftone dot area ratio of a 5% halftone dot printed portion wasmeasured using a spectrophotometer (manufactured by GretagMacbeth,product name: SpectroEye) in a measurement mode (DIN, Abs), andhighlight suitability was evaluated by the following criteria. When theevaluation is A or B, there is no problem in practical use. The resultsare shown in Table 2.

A: Halftone dot area ratio of 20% or more and less than 30%B: Halftone dot area ratio of 10% or more and less than 20%C: Halftone dot area ratio of 0% or more and less than 10%

Example 2

Printing was performed in the same manner as in the printing methoddescribed in Example 1 except that the cell depth of the gravure roll ofExample 1 was changed to 8 μm. The results are shown in Table 2.

Example 3

Printing was performed in the same manner as in the printing methoddescribed in Example 1 except that the cell depth of the gravure roll ofExample 1 was changed to 10 μm. The results are shown in Table 2.

Example 4

Printing was performed in the same manner as in the printing methoddescribed in Example 1 except that the gravure plate number and celldepth of Example 1 were changed to 250 lines/inch and 5 μm,respectively. The results are shown in Table 2.

Example 5

Printing was performed in the same manner as in the printing methoddescribed in Example 1 except that the gravure plate number of Example 1was changed to 250 lines/inch. The results are shown in Table 2.

Example 6

Printing was performed in the same manner as in the printing methoddescribed in Example 1 except that the ink 1 and gravure plate number ofExample 1 were changed to the ink 2 of Production Example 2 and 250lines/inch, respectively. The results are shown in Table 2.

Example 7

Printing was performed in the same manner as in the printing methoddescribed in Example 1 except that the ink 1 and gravure plate number ofExample 1 were changed to the ink 3 of Production Example 3 and 250lines/inch, respectively. The results are shown in Table 2.

Example 8

Printing was performed in the same manner as in the printing methoddescribed in Example 1 except that the ink 1 and gravure plate number ofExample 1 were changed to the ink 4 of Production Example 4 and 250lines/inch, respectively. The results are shown in Table 2.

Example 9

Printing was performed in the same manner as in the printing methoddescribed in Example 1 except that the ink 1 and gravure plate number ofExample 1 were changed to the ink 5 of Production Example 5 and 250lines/inch, respectively. The results are shown in Table 2.

Comparative Example 1

Printing was performed in the same manner as in the printing methoddescribed in Example 1 except that: the ink 1 of Example 1 was changedto an ink obtained by diluting and mixing 100 g of an aqueous gravureink (manufactured by Toyo Ink Co., Ltd., general-purpose aqueouslamination ink JW252 AQUAECOL F121F) with 90 g of a water/normalpropanol/isopropanol (50/25/25) diluent; and the gravure plate numberwas changed to 250 lines/inch. The results are shown in Table 2.

Comparative Example 2

Printing was performed in the same manner as in the printing methoddescribed in Example 1 except that: the ink 1 of Example 1 was changedto an ink obtained by diluting and mixing 100 g of an oil-based gravureink (manufactured by Sakata Inx Corporation, gravure ink for reverseprinting Belle Color HS R black 10000) with 30 g of a diluent(manufactured by Sakata Inx Corporation, L solvent for proof printing);and the gravure plate number was changed to 250 lines/inch. The resultsare shown in Table 2.

As apparent from Table 2, in each of Examples 1 to 9, in which theaqueous ink having a Zahn cup #3 viscosity at 20° C. of 11.0 seconds ormore and 20.0 seconds or less, and an evaporation rate of 30 mass % orless in the drying test (involving drying 1 g of the ink at atemperature of 40° C. and an air flow of 1,400 L/min for 30 minutes) wasused, and in which the gravure printing method involving transferring 1ml/m² or more and 7 ml/m² or less of the ink onto the printing mediumwas performed, the ink transfer ratio is high, and high printing densityand excellent highlight suitability are obtained.

In Comparative Examples 1 and 2, the ink transfer ratio was low, and itwas impossible to achieve both the printing density and the highlightsuitability.

1. A gravure printing method, comprising: using an aqueous ink having aZahn cup #3 viscosity at 20° C. of greater than or equal to 11.0 secondsand less than or equal to 20.0 seconds, and the aqueous ink having anevaporation rate of 30 mass % or less in a drying test, the drying testinvolving drying 1 g of the aqueous ink at a temperature of 40° C. andan air flow of 1,400 L/min for 30 minutes; and transferring equal to orgreater than 1 ml/m² and less than or equal to 7 ml/m² of the aqueousink onto a printing medium.
 2. A gravure printing method according toclaim 1, wherein a transfer ratio of the aqueous ink from a gravure cellonto the printing medium is 50% or more.
 3. A gravure printing methodaccording to claim 1, wherein a gravure cell has a volume equal to orgreater than 2 ml/m² and less than or equal to 8 ml/m².
 4. A gravureprinting method according to claim 1, wherein a gravure cell has a depthequal to or greater than 3 μm and less than or equal to 15 μm.
 5. Agravure printing method according to claim 1, wherein a gravure platenumber is equal to or greater than 150 lines/inch and less than or equalto 350 lines/inch.
 6. A gravure printing method according to claim 2,wherein a gravure cell has a volume equal to or greater than 2 ml/m² andless than or equal to 8 ml/m².
 7. A gravure printing method according toclaim 2, wherein a gravure cell has a depth equal to or greater than 3μm and less than or equal to 15 μm.
 8. A gravure printing methodaccording to claim 3, wherein a gravure cell has a depth equal to orgreater than 3 μm and less than or equal to 15 μm.
 9. A gravure printingmethod according to claim 2, wherein a gravure plate number is equal toor greater than 150 lines/inch and less than or equal to 350 lines/inch.10. A gravure printing method according to claim 3, wherein a gravureplate number is equal to or greater than 150 lines/inch and less than orequal to 350 lines/inch.
 11. A gravure printing method according toclaim 4, wherein a gravure plate number is equal to or greater than 150lines/inch and less than or equal to 350 lines/inch.